In recent years vigorous research has been performed aiming at commercialization of organic EL elements. Because high current densities can be realized at low voltages, it is anticipated that organic EL elements will achieve high emission luminance and emission efficiency. In particular, it is anticipated that high-definition multicolor organic EL displays capable of multicolor or full-color display will be commercialized.
One example of a method of realizing multicolor or full-color display in an organic EL display uses a plurality of types of color filters which transmit light in specific wavelength regions (color filter method). When employing a color filter method, it is necessary that the organic EL element emit, as the multicolor light, so-called “white light” including an appropriate balance of light in the three primary colors (red (R), green (G), blue (B)).
In order to obtain a multicolor-emission organic EL element, Japanese Patent Application Laid-open No. H3-230584 and Japanese Patent Application Laid-open No. 2000-243563 propose methods of using a plurality of emission layers including different types of emission dyes to simultaneously excite a plurality of emission dyes (see Patent References 1 and 2). And, U.S. Pat. No. 5,683,823 proposes a method of using an emission layer containing a host emission material and a guest emission material, and causing excitation of and emission by the host emission material, while simultaneously causing movement of energy to and emission by the guest material (see Patent Reference 3).
However, multicolor-emission organic EL elements rely on either simultaneous excitation of a plurality of types of emission materials, or on energy movement between a plurality of types of emission materials. It has been reported that in such elements, the balance of emission intensities among emission materials changes accompanying the passage of driving time or changes in the current passed, and there are concerns that the hue obtained may change.
As other methods of obtaining multicolor-emission organic EL elements, Japanese Patent Application Laid-open No. 2002-75643 and Japanese Patent Application Laid-open No. 2003-217859 propose color conversion methods which use single-color emission organic EL elements and color conversion layers (see Patent References 4 and 5). Color conversion layers used in these proposals are layers containing one or a plurality of color conversion materials, which absorb short-wavelength light and perform conversion into long-wavelength light.
However, if a color conversion layer is formed using a single color conversion material, a phenomenon called concentration quenching occurs. “Concentration quenching” means that energy absorbed by a color conversion material, during repeated movement between molecules, is dissipated without resulting in emission. In order to suppress this phenomenon, Japanese Patent Application Laid-open No. 2000-230172 proposes a media-disperse type color conversion layer, in which color conversion material is dissolved or dispersed in some media to lower the concentration (see Patent Reference 6). In a media-disperse type color conversion layer, as the concentration of the color conversion material is lowered, interaction between color conversion materials can be suppressed.
Here, if the concentration of the color conversion materials is lowered, the absorbance for the light to be absorbed is lowered, and so adequate converted light intensity cannot be obtained. In response to this problem, the film thickness of the color conversion layer is increased to raise the absorbance and maintain color conversion efficiency. When using such a thick color conversion layer (of film thickness approximately 10 μm), problems such as large steps, difficulty in achieving high definition, and residual water or solvent in the film, occur. When large steps exist, if an EL element is to be formed on a color conversion layer, breaks in electrode patterns occur at step portions. Further, when there is residual water or solvent in the film, upon combination with an organic EL element, the residual water or solvent causes degradation of the organic EL layer, and display defects are caused as a result.
Patent Reference 1: Japanese Patent Application Laid-open No. H3-230584
Patent Reference 2: Japanese Patent Application Laid-open No. 2000-243563
Patent Reference 3: U.S. Pat. No. 5,683,823
Patent Reference 4: Japanese Patent Application Laid-open No. 2002-75643
Patent Reference 5: Japanese Patent Application Laid-open No. 2003-217859
Patent Reference 6: Japanese Patent Application Laid-open No. 2000-230172
Patent Reference 7: Japanese Patent Application Laid-open No. H7-142169
Patent Reference 8: Japanese Patent Application Laid-open No. 2003-277743
Patent Reference 9: Japanese Patent Application Laid-open No. 2003-277744
Patent Reference 10: Japanese Patent Application Laid-open No. 2003-306454
Hence in order to realize a color conversion layer capable of maintaining adequate converted light intensity over a long period, technology to suppress concentration quenching without greatly increasing the film thickness of the color conversion layer is desired.
Further, in order to fabricate such a color conversion layer at low cost, use of an application process after dissolving the color conversion material in an appropriate solvent is desired.